Flow injection analysis (FIA) is a well known chemical analysis technique. Miniaturization of an FIA system provides additional benefits to this technique such as a reduction in consumables and generation of waste. However, miniaturization is limited, in part, by the availability of pumping systems of stable flow rates in the sub-microliter per minute domain. Thus, the fluid propulsion device is an important component in a sophisticated FIA system.
Typical fluid propulsion systems for FIA fall into one of two categories: mechanically driven or constant pressure driven. These systems include multiroller peristaltic pumps, piston (syringe type or reciprocating positive displacement type) pumps, gas-pressurized reservoirs or constant-head vessels, and piezoelectric micropumps. Unfortunately, these traditional pumping systems do not provide pumping means as reliable or affordable as desired at low flow rates.
The utilization of electroosmotic flow (EOF) as a fluid propulsion means provides a different category of pumps to the practice of FIA and other flow analysis systems. Other factors remaining the same, the magnitude and the direction of flow in an EOF pump can be readily changed by changing the current, which is linearly proportional to the volumetric flow rate, and by changing the applied voltage, which is linearly proportional to the pressure generated.
Another type of electroosmosis can also be used to generate flow which involves passing a solvated ion from one side of a compartmented chamber to another. Such electrodialysis/electroosmosis based pumps have been patented (U.S. Pat. No. 4,140,121), however, they require carefully chosen electrolytes and electrodes to eliminate gas evolution.
The feasibility of EOF pumped systems in FIA has been demonstrated using capillary electrophoresis (CE) like configurations in which-the potential is applied across the entire reaction conduit. See, for example, Liu, S.; Dasgupta, P. K. Anal. Chim. Acta 1992, 268, 1-6. Any differences in migration rates between two sequentially introduced zones lead to zone interpenetration that can be carried out in a controlled manner. Any combination of adjacent zones can be mixed in this fashion, unless the species of interest in both such zones are electrically uncharged. Advantages of utilizing an EOF pump in this format include: analyte enrichment by electrostacking, assuming conductance of the sample solution is significantly lower than that of the reagent carrier solution; limited dispersion despite long reaction times and continuous flow; and separation of different reactive constituents of the sample and/or separation of the sample matrix from the constituent of interest.
Despite the advantages of using EOF pumped FIA systems in a CE-like configuration, the choice of the composition of the reaction system is limited because the composition must be such that EOF is generated at the desired level in the desired direction. Moreover, typically multiple line schemes are necessary in FIA, whereas a CE-like configuration is a single line scheme. Consequently, the CE-like configuration is inapplicable in many analytical applications, e.g., when the reaction must be carried out at very high or very low pH, in highly conductive saline solutions, or in a nonconducting organic medium. In all of these cases, electrical current levels are either excessive or inadequate to support significant EOF. Further, EOF has a flat profile, and the dispersion and dilution associated with a parabolic flow profile is desired, even necessary, in some FIA applications such as on-line dilution, gradient dilution techniques using a single standard, and FIA titration.
The art of FIA analysis would be improved by a reliable fluid propulsion device capable of handling low flow rates such that an overall miniaturization of the FIA system may be achieved; this device would preferably permit multiple line schemes, and would not significantly limit choice in composition of the reaction system.